1. Field of the Invention
The present invention relates to a technique for forming a color image with excellent color development and color uniformity Particularly, the present invention relates to an ink set comprising a liquid composition and an aqueous color ink suitable for image formation by an ink-jet recording system, a method for forming a colored area on a recording medium; and an ink-jet recording apparatus employing the ink set.
2. Related Background Art
The ink-jet recording system conducts recording by ejecting an ink onto a recording medium like a paper sheet. For example, ink-jet recording systems which employ an electro-thermal transducer as an ejection energy supplying means and eject liquid droplets by forming bubbles in the ink with thermal energy applied to the ink are disclosed in Japanese Patent Publication Nos. 61-59911, 61-59912, and 61-59914. In such an ink-jet recording system, the recording head orifice can readily be multiplied in a high density to obtain images with high resolution and high quality at a high speed.
In conventional ink-jet recording systems, the ink is mainly composed of water and contains a water-soluble high-boiling solvent such as glycol to prevent drying of ink in the nozzle and clogging of the nozzle, and for other purposes. Such an ink may cause problems of incomplete fixation of the ink, or nonuniformity of the formed image presumably caused by nonuniform distribution of a filler or a sizing agent on the face of a recording medium paper sheet. On the other hand, in recent years, the ink-jet recorded image is required to have a high image quality comparable to that of silver salt photograph and greatly demanded technically to give a higher image density and a broader color reproduction range of the ink-jet recorded image, and higher uniformity of the color of the recorded image
In such circumstances, many methods are proposed for stabilization of the ink-jet recording and improvement of the quality of ink-jet recorded matters One of the proposals concerning the recording medium is a method of coating the base paper of the recording medium with a filler or a sizing agent on the surface thereof. For example, porous fine particles capable of adsorbing a coloring material are applied as a filler on the base paper to form an ink-receiving layer. Coated paper for ink-jet recording is commercially available as the recording medium produced by employing such techniques.
Other typical methods are summarized below.
(1) Incorporation of a Volatile Solvent, or a Penetrative Solvent Into an Ink:
Japanese Patent Application Laid-Open No. 55-65269 discloses adding a compound which promotes penetration of a surfactant or the like to the ink for accelerating fixation of the ink on the recording medium. Japanese Patent Application Laid-Open No. 55-66976 discloses using an ink mainly composed of a volatile solvent
(2) Mixing of an Ink with a Liquid Composition Reactive to the Ink on a Recording Medium.
An image-improving liquid composition is applied on a recording medium, before or after application of a recording ink, to improve image density and water resistance and to prevent bleeding.
Japanese Patent Application Laid-Open No. 63-60783 discloses preliminary application of a basic polymer-containing liquid composition and subsequent recording with an ink containing an anionic dye. Japanese Patent Application Laid-Open No. 63-22681 discloses a recording method in which a first liquid composition containing a reactive chemical species and a second liquid composition containing a compound capable of reacting with the reactive chemical species are mixed on a recording medium. Japanese Patent Application Laid-Open No. 63-299971 discloses preliminary application of a liquid composition containing an organic compound having two or more cationic groups per molecule onto a recording medium and subsequent recording with an ink containing an anionic dye. Japanese Patent Application Laid-Open No. 64-9279 discloses a recording method in which an acidic liquid composition containing succinic acid or the like is applied onto a recording medium and subsequently recording is conducted with an ink containing an anionic dye.
Japanese Patent Application Laid-Open No. 64-63185 discloses application of a liquid composition capable of insolubilizing an ink dye before recording with an ink. Japanese Patent Application Laid-Open No. 8-224955 discloses using a liquid composition containing cationic substances of different molecular distribution regions in combination with an ink containing an anionic compound. Japanese Patent Application Laid-Open No. 8-72393 discloses using a liquid composition containing a cationic substance and finely pulverized cellulose in combination with an ink. These methods are said to give good images which are high in the image density, better in the print quality and water resistance, and also better in the color reproducibility and non-bleeding. Japanese Patent Application Laid-Open No. 55-150396 discloses, after recording with a dye ink on a recording medium, applying a water resistance-imparting agent which forms a lake by reaction with the dye to improve the water resistance of a recorded image.
(3) Mixing of an Ink with a Fine Particle-containing Liquid Composition on a Recording Medium:
Japanese Patent Application Laid-Open No. 4-259590 discloses applying a colorless liquid containing colorless inorganic fine particles on a recording medium and subsequently applying a nonaqueous recording liquid thereto. Japanese Patent Application Laid-Open No. 6-92010 discloses a method in which a solution containing fine particles or a solution containing fine particles and a binder polymer is first applied to a recording medium and subsequently an ink containing a pigment, a water-soluble resin, a water-soluble solvent and water is applied thereto. These methods are said to give images with better print quality and color development, irrespective of the kind of paper used.
The inventors of the present invention, after comprehensive investigation on various ink-jet recording techniques as mentioned above, have found that the above described methods achieve remarkable effects in solving the respective technical problems, but other important ink-jet recording properties may be impaired instead. For example, the above technique (1) improves fixation of an ink on a recording medium, but the Image density may become deteriorated, and the color reproduction range which is important in recording on plain paper or recording a color image may become insufficient. The above technique (2) gives high image density owing to the coloring material of the ink made to remain on the surface of a recording medium, but the color reproduction range or color chroma may be lowered, presumably owing to aggregation of the coloring material on the surface of the recording medium.
The aforementioned recording medium having a base paper the surface of which is coated with a filler or a sizing agent (hereinafter referred to as xe2x80x9ccoated paperxe2x80x9d) is recognized to be capable of forming a high-quality image.
Generally, for obtaining an image of high chroma, it is known that the coloring material should be left on the surface of a recording medium in a monomolecular state without being aggregated. The fine porous particles of the coated paper serve surely to make the coloring material to remain on the recording medium surface. However, for achieving a high image density and high image chroma, a larger amount of the porous fine particles is required to form an ink-receiving layer with a thickness enough to cover the base paper, which gives rise to a disadvantageous problem that the feel or texture of the base paper sheet is lost. The inventors have presumed that such a thick ink-receiving layer is required because the coloring material is not effectively adsorbed to the porous fine particles.
For the explanation below, a coated paper is taken which has a single ink-receiving layer. FIG. 9 shows schematically a cross section of the surface and its vicinity of the coated paper. In FIG. 9, the numeral 901 denotes a base paper sheet, and the numeral 903 denotes an ink-receiving layer. The ink receiving layer 903 contains porous fine particles 905 and an adhesive 907 for fixing the porous fine particles. The ink applied on the paper penetrates into interstices or air gaps between the porous fine particles 905 by capillarity to form ink penetration regions 909. Since the density of the porous fine particles in the ink-receiving layer differs with the places as shown in FIG. 9, the way of penetration of the ink by the capillarity also varies depending on the places. Therefore, in the process of penetration of the ink, the colorant is not brought Into uniform contact with the surface of the porous fine particles, resulting in ineffective adsorption of the colorant by the porous fine particles.
Moreover, the adhesive 907 may hinder the penetration of the ink in some portions, and those portions to which the ink cannot penetrate may be present in the ink receiving layer, and in this case, portions may be formed which do not take part in the color development.
From the above reasons, in the conventional coated paper, the porous fine particles cannot effectively adsorb the coloring material in a monomolecular state for the amount of the particles used. Therefore, a larger amount of the porous fine particles should be used for obtaining a high-quality image, which impairs the feel of the base paper.
Based on the new knowledge as mentioned above, the inventors have found that the coloring material and the fine particles are caused to react with each other in a liquid-liquid state by using the fine particles capable of adsorbing the coloring material and, to cause the fine particles to effectively adsorb or bond the coloring material, dispersing the fine particles in a liquid phase for use in conjugation with the ink in a liquid state, and as a result, the density and chroma of the images can be improved.
Another technical problem in ink-jet recording is bleeding. In formation of multicolor images by the ink-jet recording, a plurality of inks which different colors must be applied in superposition or in adjacency on a recording material. In a boundary between the adjacent different color areas, the colors may feather or bleed and cause nonuniform mixing to lower the image quality. Hence, how the bleeding should be prevented is also an important technical problem in providing multi-color images with high quality.
The inventors have made comprehensive investigation to solve the problem of bleeding on the basis of the aforementioned technique of improving the image density and the color chroma by effective adsorption or bonding of the coloring material to the fine particles, and consequently completed the present invention
An object of the present invention is to provide an ink set for obtaining a higher quality, ink-jet record with a high density and a high chrome of the image and with sufficiently controlled bleeding.
Another object of the present invention is to provide a method of forming a colored area on a recording medium with a broad color reproduction range, excellent uniformity of the colors, effectively suppressed bleeding at the boundary between adjacent different color regions on the recording medium, and excellent rub-off resistance of the colored area.
Still another object of the present invention is to provide a method capable of forming a colored area on a recording medium with a broad color reproduction range, excellent uniformity of the colors, effectively controlled bleeding at the boundary between adjacent different color regions on the recording medium, and high rub-off resistance of the image.
A further object of the present invention is to provide an ink-jet recording apparatus which can form colored area on a recording medium with a broad color reproduction range, excellent uniformity of the colors, effectively suppressed bleeding at the boundary between adjacent different color regions, and excellent rub-off resistance of the image.
A still further object of the present invention is to provide an ink set which is excellent in the shelf stability, ejection stability from a recording head, and ink-jet recording properties.
According to an aspect of the present invention, there is provided an ink set comprising separately an aqueous anionic or cationic ink containing a coloring material, and a liquid composition containing fine particles dispersed therein and electrically charged at the surface in a polarity opposite to the ink, wherein the ink and the liquid composition are adjusted to give, when mixed at a mixing weight ratio of about 1:1, a pH of the liquid mixture to cause the aggregation of at least the coloring material.
According to another aspect of the present invention, there is provided an ink set comprising separately an aqueous anionic or cationic ink containing a coloring material, and a liquid composition containing fine particles dispersed therein and electrically charged at the surface in a polarity opposite to the ink, wherein the pH of the ink and the pH of the liquid composition are adjusted respectively to give, when mixed at a mixing weight ratio of about 1:1, a pH of the liquid mixture to cause aggregation of the coloring material.
According to a further aspect of the present invention, there is provided an ink set comprising separately an anionic or cationic ink containing a coloring material, and a liquid composition containing fine particles reactive to the coloring material in a dispersion state, wherein the fine particles in the liquid composition are capable of adsorbing or bonding the coloring material in the ink on the surface of the fine particles with the coloring material maintaining the molecular state the coloring material has had in the ink when the ink and the liquid composition are mixed in a liquid state, and the ink and the liquid composition are adjusted to give, when mixed for a liquid-liquid reaction, a pH of the liquid mixture to cause the aggregation of the coloring material.
According to still another aspect of the present invention, there is provided an ink set comprising separately an anionic or cationic ink containing a coloring material, and a liquid composition containing fine particles reactive to the coloring material in a dispersion state, wherein the fine particles in the liquid composition are capable of adsorbing or bonding the coloring material in the ink on the surface of the fine particles with the coloring material maintaining the molecular state the coloring material has had in the ink when the ink and the liquid composition are mixed in a liquid state, thereby making the dispersion state unstable to cause the aggregation of the fine particles, and the ink and the liquid composition are adjusted to give, when mixed for a liquid-liquid reaction, a pH of the liquid mixture to cause the aggregation of the coloring material.
According to a still further aspect of the present invention, there is provided an ink set comprising separately an anionic or cationic ink containing a coloring material, and a liquid composition containing fine particles reactive to the coloring material in a dispersion state, wherein the fine particles in the liquid composition are capable of adsorbing the coloring material in the ink in a monomolecular state on the surface of the fine particles when the ink and the liquid composition are mixed in a liquid state, and the ink and the liquid composition are adjusted to give, when mixed for a liquid-liquid reaction, a pH of the liquid mixture to cause the aggregation of the coloring material.
According to still another aspect of the present invention, there is provided a method of forming a colored area on a recording medium comprising a step of ejecting a droplet of an aqueous anionic or cationic ink and a droplet of a liquid composition to the recording medium in such a way that both of the droplets are brought into contact with each other in a liquid-liquid state on the recording medium, said aqueous ink containing a coloring material, said liquid composition containing fine particles dispersed therein and electrically charged at the surface in a polarity opposite to the ink, wherein the aqueous ink and the liquid composition are adjusted to give, when mixed at a mixing weight ratio of about 1:1, a pH of the liquid mixture to cause the aggregation of at least the coloring material.
According to a still further aspect of the present invention, there is provided an ink-jet recording apparatus comprising an ink container containing an aqueous anionic or cationic ink containing a coloring material, a liquid composition container containing a liquid composition containing fine particles dispersed therein and electrically charged at the surface in a polarity opposite to the ink, and heads for separately ejecting the liquid composition and the aqueous ink.
According to still another aspect of the present invention, there is provided an image comprising a colored area, the colored area being formed by a liquid-liquid reaction of an anionic or cationic ink containing a coloring material with a liquid composition containing fine particles reactive to the coloring material in a dispersion state, wherein the colored area contains the fine particles having the coloring material adsorbed or bonded in a monomolecular state on the surface thereof, and an aggregate of the coloring material.